Method of making a sheathed electric heating unit



R. D. BAXTER 3,234,633

METHOD OF MAKING A SHEATHED ELECTRIC HEATING UNIT Feb. 15, 1966 FiledSept. 28, 1962 RR 0E TT NVA EA VB mD T. R E B 0 Dn HIS ATTORNEY UnitedStates Patent Utilice 3,234,633 Patented Feb. 15, 1966 3,234,633 METHODF MAKING A SHEA'IHED ELECTRIC HEATING UNIT Robert D. Baxter,vShelbyville, Ind., assigner to General Electric-Company, a corporationof New York iFiled Sept. 28, 1962, Ser. No. 226,927 4 Claims. (Cl.29--155.64)

This invention relates to a sheathed electric heating unit and to amethod of making the same. It is particularly applicable to glow plugswhich are commonly used in ignition systems such as, for example, in theignition systems of diesel engines.

In the typical glow plug design, a heating element, usually in the formof a coil of resistance wire, is imbedded in compacted insulatingmaterial Within an outer metallic sheath. One end of the heating coil iselectrically connected to the tip end of thesheath, typically by weldingthe end of the coil to the sheath at that point, and the other end ofthe coil is electrically connected to a terminal which extends from thesheath at the end of the plug opposite the tip.

It is important in such a unit from the standpoint of ensuring adequateoperating life that the coil of resistance wire be properly centered inthe sheath to prevent the development of a short circuit between thecoil and the sheath, and also that the diameter of the wire be selectedin light of the other` design parameters to avoid operation of the coilat excessively high temperatures. In general, it is desirable that thediameter of the resistance wire be as large as possible within thephysical confines of the design, taking into account, of course, theother design parameters involved. For a given -heat dissipation rate, alarger wire diameterprovides a greater wire surface area and thereforereduces the heat dissipation rate per unit surface area of the wire.This permits the attainment of a longer operating life for the unit.

In addition to the foregoing factors, it is also important that thespacing between adjacent turns of the heating coil be uniform along thelength of the sheath, particularly near the tip end of the plug. Unevenspacing of the coil turns causes some portions of the unit to run hotterand others to run cooler than the desired temperature, although aslightly closer coil spacing may be desirable near the terminal end of aglow plug because in the typical installation some of the heat isusually conducted away from the plug through the terminal end. It isparticularly important, however, to avoid excessive spacing near the tipof the plug because this causes the tip or working end of the plug torun cooler than the rest of the unit.

A typical method which `has been heretofore used to manufacture glowplugs involves welding a coil of resistance wire to one end of a sheath,attaching a terminal to the opposite end of the coil and then stretchingthe coil longitudinally within the sheath to center it while granulatedinsulating material is loaded into the sheath.

, Stretching of the coil permits it to be held in avcenter position inthe sheath and it alsoallows the granulated insulating material to flowinto the center of the coil through the resulting spaces ybetween thecoil turns.

The welding of the coil to the sheath, howeveranneals and softens thatend of the coil so that the above-mentioned stretching technique causesthe coil to stretch by a greater amount near the tip end of the plug,thus producing a greater spacing 'between adjacent coil turns andcausing the tip end of the plug to run cooler than the rest of the unit.In addition, the requirement for stretching reduces the diameter oftheresistance wire which can be accommodated because the spaces between theadjacent coil Vturns produced by the stretching represent spaces whichcould at least partially be occupied by wire of a greater diameter.

Attempts to avoid these problems by `reducing the amount of tensionapplied -to-the'terminal1have,lhowever, met with diiiiculties becausethe problem of centering the coil in sheath becomes more acute and the-reducedspacing between t-he adjacent coil turns impedes the ow ofgranulated insulating material -into the centerof-the coil. Attempts tocompromise between theforegoing factors have produced difficulties withrespect to cooler than desired operating temperatures near the tip ofthe plug, adequate centering of the coil Vinthe sheath and theattainment of proper operating life through the use of optimumresistance wire diameters.

Accordingly, it is an object of this invention to provide an improvedsheathed heater of the glow plug type having an increased operatinglife. i

It is another object of this invention to provide an improved sheathedheater of the yglow plug type having a more uniform spacing between coilturns, particularly near the-tip end of the unit, while retainingaccurate centering ofthe coil within the sheath.

It is still another-object of this inventiontoprovide an improved methodof manufacturing a sheathed heater of the glow plug type which avoidslthe-necessity for stretching the heating coil or displacing adjacentturns of the coil from each other during the load-ing of insulatingmaterial into the unit.

Brietiy stated, these and other objects of this invention areaccomplished, in one'embodimentthereof, by placing a frangible tube vofsolid insulating material between the outer diameter of a heating coiland the inner -wall of a sheath to center the coil inthe sheath. lGranulated insulating material is then loaded into the center of thecoil through a duct, which in one embodiment extends through a -hollowterminal which is connected -to the coil. The frangible tubelofinsulating material Ais Athen crushed and compacted along with thegranular insulating material in the final compression of the unit.

Because stretching of the coil isvnot required, a more uniform spacingbetween the coil turns, Vparticularly near the tip end of the unit, isobtained. In addition, since the granulated insulating `material is notrequired to ow between the adjacent coil turns into the center of thecoil, a closely wound coil may be used, thuspermitting the utilizationof a maximum wire diameter.

For a thorough understanding of this invention, reference may be had tothe following description, taken in connection with the accompanyingldrawing in which:

FIG. l is a cross-sectional view of a glow plug, at an intermediatestage of manufacture, embodying this invention and illustrating theinitial steps of the method of this invention.

FIG. 2 is a cross-sectional view of the glow plug of FIG. 1 at asubsequent stage of manufacture and during the insulation loadingoperation.

FIG. 3 shows the glow plug of FIGS.-1 and 2 in its completed form.

FIG. 4 shows, in vertical cross-section,a glow plug similar to that ofFIGS. 1-3 but embodying a modified terminal arrangement.

FIG. 5 shows `an enlarged coil-end view of the terminal of FIG. 4.

FIG. 6 shows, in enlarged front view, a second modified terminalstructure.

FIG. 7 shows an enlarged coil-end view of the terminal of FIG. 6.

Referring to the drawing, there is shown in FIG. l a partially completedglow plug 10 of this invention prior to the process step in whichpowdered insulating material is loaded into the unit. At this stage ,ofthe manufacturing process the `glow plug assembly 10 comprises an outersheath 14, normally constructed of metal or other electricallyconductive material, in which is disposed a heater assembly showngenerally at 15. The heater assembly comprises a heating element in theform of a coil 16 of resistance wire and a terminal 13 which is attachedto one end 16a of the coil 16. A tube 18 of frangible insulatingmaterial is sl'idably mounted between the inner wall of the sheath 14and the outer diameter of the coil 16 to center the coil in the sheath.The tube 18 may be of any suitable insulating material, such asmagnesium oxide.

A duct 12 extends through the terminal 13 and is -shown in FIGS. 1-3 inthe preferred form of a passageway extending through the center of theterminal 13. In other words, the terminal 13 is hollow or tubular. Theduct 12 provides a communicating passageway between the exterior of thecoil 16 and its central or core portion through one end of the coil and,as will be explained later on in greater detail, the duct 12 may takeother forms than the particular one illustrated.

To reach the point in the assembly shown in FIG. 1, the terminal 13 isattached to the upper end of the coil 16a lby welding or in any othersuitable manner and the assembly thus formed is then inserted in theopen end of the sheath 14. The sheath is crimped as shown at 19 to holdthe lower end 16b of the coil tight and flush with respect to the end14b of the sheath. The tube 18 of insulating material is placed in thesheath as shown in FIG. l to center the coil in the sheath. The lowerend of the tube 18 bears against the reduced diameter portion of thecrimped section 19 of the sheath.

At this point in the process, the lower end 14h of the sheath is weldedclosed, simultaneously joining the end 1612 of the coil to the sheath soas to make an electrical connection therebetween as can be seen in FIG.2. The steps of connecting the lower end 16b of the coil to the sheathand of closing the lower end of the sheath may of course Iheaccomplished in any suitable fashion although I have found it convenientto accomplish both of these steps with a single welding operation asdescribed above.

With the coil 16 remaining in its normal unextended position, accuratelycentered by the tube 18 of compressed insulating material, powderedinsulating material 20 is introduced into the interior of the coil 16through the top or open end of the duct 12 in the terminal 13. For

satisfactory loading and subsequent compacting, the heating unit ispreferably vibrated during the loading process. Any suitable powderedmaterial may be used, magnesium oxide being very satisfactory. It hasybeen found desirable to ll the interior of theduct 12 with powderedinsulation also since this prevents a void through which the powderedinsulating material could otherwise iiow from the center ofthe coilduring subsequent swaging and compression of the sheath. The space atthe top of the coil between the terminal 13 and the tube of insulatingmaterial 18 is also filled with powdered insulating material.

A closure member, which is shown in FIG. 2 in the form of a disk 21 ofinsulating material, may then he inserted in the open end 14a of thesheath 14 although it will -be appreciated that with the duct 12 in theform shown in FIGS. l-3, the loading of the powdered insulating materialthrough the duct 12 may be accomplished with the disk 21 in place. disk21 may be utilized to center the terminal 13 during the loadingoperation.

To complete the production of the heating unit, the upper end of thesheath is bent at 22, as shown in FIG. 3, to hold the disk 21 firmly inplace and lthe upper portion 23 of the terminal 13 is crimped or pluggedto prevent loss of insulating material from the unit. The unit is thenswaged or otherwise compressed down to its linal dimensions.

The final swaging or compression operation crushes the tube ofinsulating material 18 and compacts this crushed insulation along withthe powdered'insulating material With this latter approach, the

which has been loaded into the sheath. In the production of the heatershown in FIG. 3, the diameter of the unit is reduced to two differentdimensions. The portion 10a which includes the coil 16 is reduced to asmaller diameter than the remaining portion 10b. It should be noted atthis point that the hollow terminal 13 provides a secondary advantage inthat the terminal is squeezed down to a smaller diameter along with thecoil 16 to provide additional radial clearance between the upper end ofthe coil and the sheath than would otherwise be available with a solidterminal where no similar reduction in diameter would occur. In the pastit has been found necessary in some designs to utilize a smallerdiameter wire around the terminal in order to avoid the problem ofreduced radial clearance between the coil and the sheath at this pointybut the adaptability of the hollow terminal to a reduction in diameteravoids this necessity.

The compression and reduction of diameter of the unit elongates andspaces the turns of the coil 16. The turns are uniformly spaced exceptthat a slightly closer spacing is obtained near the coil-terminal jointbecause of the transition to the upper area where the degree ofcornpression and elongation is less than at the lower end of the unit.This closer spacing at the coil-terminal joint has been found to beadvantageous in that it increases the density of heat generation at thisend of the unit where in the typical installation so-me of the heat isusually conducted away from the plug through the terminal end.

FIGS. 4 through 7 reveal twoy modified terminals providing ducts throughwhich powdered insulating material may be introduced int-o the interiorof the heating coil unit. In FIG. 4 a solid rod-type terminal 33 havinga duct or keyway 34 formed therein is shown aflixed to one end of a coil36. This modification is more clearly shown in the coil end View of theterminal 33 in FIG. 5. The slot 34 forms a passageway communicating withthe interior of the coil 36 through the upper end of the coil andthrough which powdered insulating material may be loaded into the centerof the coil.

Another alternative arrangement is shown in FIGS. 6 and 7. In thisembodiment the coil end of a rod-type terminal 43 is provided with aduct or slot 44 which forms a passageway through which insulatingmaterial may flow into the interior of the heating coil.

It should be noted that in the embodiments of FIGS. 4 through `6 theducts which communicate with the center of the coil do not extendexternally from the sheath and, accordingly, need not be externallyclosed off prior to the nal swaging operation. On the other hand, withthis type of duct structure, lth-e loading of the insulating materialinto the center of the coil must be accomplished before the open end ofthe sheath is closed off. Such alternative duct arrangements may beextended, of course, beyond the open end of the sheath but the extendedduct portion should then be closed olf or suitably plugged before thenal swaging operation as is done in the case of the embodiment shown inFIGS. l-3.

It will be observed that the foregoing method avoids the necessity forstretching the heating coil to center it in the sheath during theloading operation. Accordingly, the problem of excessive stretching ofthe coil near the welded end is avoided. In addition, it will be notedthat with the use of duct means communicating with the center of -thecoil through one end thereof to accommodate the loading of the powderedinsulating material into the center of the coil', the need forstretching the coil to permit the insulating material to flow betweenadjacent turns of the coil is also eliminated. Such a techniquetherefore permits the use of a closely wound coil with the turns of thecoil .being closely adjacent or even touching each other in theunstretched state, thereby allowing the use of a maximum resistance wirediameter. In other words, the use of a space wound coil, that is, a coilin which the turns are spaced relatively far apart in the unstretchedstate to permit insulating material to llow into the center of the coilthrough the spaces between the turns, is not required.

It will thus be observed that my invention produces an improved heatingunit in which the heating coil is accurately centered within the sheathwith the adjacent turns of the heating coil being more uniformly spacedand with a greater amount of heating wire being accommodated in theunit. The foregoing advantages take the form of a heating unit ofsubstantially improved quality with a significantly increased operatinglife capability.

It will be appreciated, of course, that various modifications may bemade in the structures and in the process steps which I have described.For example, the tube 18 of insulating material may be made up of anumber of separate elements and may be in a different form than theparticular one which I have illustrated. In addition, variousalternative methods of securing the terminal to the heating coil and ofclosing the ends of the sheath may be employed. The duct whichcommunicates with the center of the coil to accommodate the loading ofthe powdered insulating material into that area may, of course, takevarious forms and I have illustrated several alternative approaches inthis regard. Another approach would be to provide duct means into thecoil through one end thereof independently of the terminal structure,although I prefer to combine the duct with the terminal.

Thus, while I have described several embodiments of my invention inconsiderable `detail it should be appreciated that I have done this forpurposes of presenting a full and clear description and that myinvention is not limited to the particular details which I havepresented. Accordingly, it is to be understood that various changes,modifications and substitutions lmay be made in the subject matterpresented herein without departing from the true scope and spirit of myinvention as I have defined it in the appending claims.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A method of making a sheathed heating unit comprising the steps of:

(a) attaching a generally hollow open-ended unextended helical heatingelement to a terminal having a duct which communicates with the interiorof the element to form a heater assembly,

(b) inserting said assembly in a sheath,

(c) placing at least a portion of a frangible cylindrical -member ofinsulating material between the inner wall of the sheath and the heatingelement to center the heating element with respect to the sheath,

(d) closing one end of said sheath,

(e) introducing insulating material through said duct of said terminalinto the interior of said element,

(f) closing the other end of said sheath, and

(g) compressing the unit to extend said heatin-g element and compactsaid insulating material.

2. A method of making a sheathed heating unit comprising the the stepsof (a) attaching one end of an unextended coil to a terminal having aduct which communicates with the interior of said coil to form a heaterassembly,

(b) inserting said assembly in a sheath,

(c) placing at least a portion of one frangible member of compressedinsulating material between the coil and the inner wall of the sheath tocenter the coil in the sheath,

(d) closing one end of said sheath while simultaneously joining the freeend of said coil to said one end of said sheath,

(e) introducing insulating material through said duct of said terminalinto the interior of said coil,

(f) closing the other end of said sheath, and

(g) compressing the unit to extend said coil and compact said insulatingmaterial.

3. A method of making a sheathed heating unit cornprising the steps of:

(a) attaching one end of an unextended coil to a terminal having a ductwhich communicates with the interior of said coil to form a heaterassembly,

(b) inserting said assembly in a sheath so that the other end of saidcoil is adjacent one end of said sheath,

(c) inserting at least one frangible member of cornpressed insulatingmaterial -between said coil and the inner wall of said sheath so as tosurround at least a portion of said coil,

(d) closing said one end of said sheath while simultaneously joiningsaid other end of said coil to said one end of said sheath,

(e) introducing insulating material through said duct of said terminalinto the interior of said coil,

(f) closing the other end of said sheath,

(ig) closing one end of said duct, and

(h) compressing the unit to extend said coil and crush said member ofinsulating material and compacting all of said insulating material.

4. A Imethod of making a sheathed heating unit comprising the steps of:

(a) attaching one end of an unextended coil to a tubular terminal toform a heater assembly,

(b) inserting said assembly in a sheath so that the other end of saidcoil is Iadjacent one end of said sheath,

(c) inserting at least one frangible member of cornpressed insulatingmaterial between said coil and the inner wall of said sheath so as tosurround at least a portion of said coil,

(d) closing said one end of said sheath while simultaneously joiningsaid other end of said coil to said one end of said sheath,

(e) introducing insulating material through said terminal into theinterior of said coil,

(f) closing the other end of said sheath,

(g) compressing the unit to extend the coil, to reduce the heating coilportion of said unit to one dimension and to reduce the remainder ofsaid unit to a second, greater, dimension whereby said member ofinsulating material is crushed, all of said insulating material iscompacted and the porti-on of said terminal attached to said coil isconstricted to increase the clearance between said terminal and theinner Wall of said sheath.

2/1937 Great Britain.

10/ 1938 Great Britain.

JOHN F. CAMPBELL, Primary Examiner.

RICHARD M. WOOD, Examiner.

70 H. T. POWELL, J. W. BOCK, Assistant Examiners,

1. A METHOD OF MAKING A SHEATHED HEATING UNIT COMPRISING THE STEPS OF:(A) ATTACHING A GENERALLY HOLLOW OPEN-ENDED UNEXTENDED HELICAL HEATINGELEMENT TO A TERMINAL HAVING A DUCT WHICH COMMUNICATES WITH THE INTERIOROF THE ELEMENT TO FORM A HEATER ASSEMBLY, (B) INSERTING SAID ASSEMBLY INA SHEATH, (C) PLACING AT LEAST A PORTION OF A FRANGIBLE CYLINDRICALMEMBER OF INSULATING MATERIAL BETWEEN THE INNER WALL OF THE SHEATH ANDTHE HEATING ELEMENT TO CENTER THE HEATING ELEMENT WITH RESPECT TO THESHEATH, (D) CLOSING ONE END OF SAID SHEATH, (E) INTRODUCING INSULATINGMATERIAL THROUGH SAID DUCT OF SAID TERMINAL INTO THE INTERIOR OF SAIDELEMENT, (F) CLOSING THE OTHER END OF SAID SHEATH, AND (G) COMPRESSINGTHE UNIT TO EXTEND SAID HEATING ELEMENT AND COMPACT SAID INSULATINGMATERIAL.